Stock structure of a coral reef fish, Plectropomus leopardus: identification and implications for harvest strategy evaluation

Bergenius, Mikaela A.J. (2007) Stock structure of a coral reef fish, Plectropomus leopardus: identification and implications for harvest strategy evaluation. PhD thesis, James Cook University.

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Abstract

The recognition of stocks, or spatially separate groups of individuals with persistent differences in biological characteristics, is important for the sustainable and optimised use of fisheries resources. Stocks with different biological characteristics may respond differently to harvest and therefore have different vulnerabilities to over-exploitation. Recent research suggests that spatial variation in biological characteristics at a range of scales is a feature of many coral reef fishes. An investigation of the temporal stability of spatial differences and the identification of stocks that may require separate management strategies, however, has rarely been undertaken for such fish. Moreover, little is known about the implications of spatial differences in biological characteristics for spatially separate components as well as entire populations of coral reef fishes when subject to various levels of harvest.

This thesis has four primary aims: 1) to investigate the use of otolith chemistry as an indirect indicator of stock structure of three exploited epinepheline serranid coral reef fishes, Plectropomus leopardus, Cephalopholis cyanostigma and Epinephelus fasciatus, on the Great Barrier Reef (GBR), Australia; 2) to investigate the use of otolith morphology as an indirect indicator of stock structure of P. leopardus; 3) to investigate the spatial and temporal patterns in life history characteristics of P. leopardus as direct manifestations of stock structure; and 4) to examine the implications of spatial variability in life history characteristics of P. leopardus for harvest strategy evaluation.

The first two aims were achieved by comparing otolith chemistry (Mn, Sr, Ba) and otolith morphology (otolith length, width, area, perimeter, circularity, rectangularity and Fourier Harmonics) variables among four regions of the GBR, separated by 100s of kilometres, as well as among three reefs 100s – 1000s of meters apart within each region. The temporal stability in otolith chemistry and morphology signals was also examined by comparing two cohorts of P. leopardus and individuals of C. cyanostigma and E. fasciatus from the same cohorts that were collected two years before and two years after a significant weather disturbance (Tropical Cyclone Justin) in March 1997. Persistent differences in otolith chemistry were found at both broad and fine spatial scales, and differences in otolith morphology were seen at mainly broad spatial scales. Moreover, some aspects of chemical and morphological signals differed between cohorts and individuals collected before and after the Cyclone. The results highlight the need to incorporate data from several years in studies using these techniques to discriminate temporary and possibly misleading signals from those that indicate persistent spatial structure in stocks. These results provide a good starting point for future research on groups of individuals that have lived at least part of their lives in different environments and therefore may have different biological characteristics, although otolith chemistry and morphology should not be used in isolation to determine stock structure.

The third aim was achieved by comparing vital life history characteristics of P. leopardus at the same broad regional and finer reef scale as the otolith chemistry and morphology. Temporal stability in stock structure was examined by comparing biological parameters among five consecutive years, from 1995 to 1999. The results matched the patterns indicated by otolith chemistry and morphology and emphasised that the stock structure of P. leopardus is far more complex than assumed previously. Mortality, growth, age richness and longevity of P. leopardus varied among reefs within regions and some estimates of growth and longevity also varied at the larger regional spatial scale. Several of the spatial patterns in these biological parameters were complicated by inter-annual variation. Similar to the use of otolith chemistry and morphology, the life history results emphasised the importance of a multi-scaled sampling design, including a temporal component, when using biological characteristics to investigate the stock structure of tropical reef fishes. I proposed a theoretical model for conceptualising the stock identification and management challenge for P. leopardus that may be composed of a complex network of reef subpopulations, groups of subpopulations (i.e., regions), and potential stocks with persistent differences in biology.

The fourth aim was achieved using a spatially-structured management strategy evaluation model developed for P. leopardus harvested by the GBR line fishery. Relative spawning biomass of the population was estimated from simulations of four hypothetical scenarios of spatial variation in life history characteristics under each of five hypothetical effort scenarios. The life history scenarios involved simulating the P. leopardus population with or without differences in a) individual growth and b) mortality rates among four regions of the GBR. The different effort scenarios involved shifting fishing effort among the four regions. The effects of regional closures (no effort) were also examined. Trajectories of mean relative spawning biomass were compared among the different combinations of spatial and effort scenarios. Relative mean spawning biomass trajectories were also compared between two analytical approaches involving aggregating results over regions, as is usually done in fisheries assessments, or treating results separately for each region. The latter comparison directly assessed the impacts of erroneously assuming an homogeneous stock despite regional variation in life history parameters.

Including spatial variation in growth and mortality resulted in greater depletions of relative spawning biomass and longer times to recover relative to exploitation levels for the population as a whole, as well as for several regions. Aggregating results across regions masked important region-specific patterns in the relative spawning biomass trajectories arising from spatial variation in biology, and so resulted in the wrong conclusions about whether particular management objectives were likely to be realised. These results suggest that spatial variation in growth, in particular, mortality and potentially other life history characteristics should be incorporated in future harvest strategy evaluations for P. leopardus. Further, the results suggest that the single management unit currently in place for P. leopardus on the GBR may need to be divided into finer spatial units to closer reflect biological stock units to deliver prudent biologically optimal harvests.

The results clearly have some important implications for the management and harvest of P. leopardus on the GBR. P. leopardus, and probably many other exploited coral reef fishes, should not be viewed or managed as single homogenous populations. Instead these populations should be considered as complex networks of spatially and temporally varying components which although interlinked, may require separate management strategies to assure their long-term sustainability and optimal harvest.

Item ID: 2008
Item Type: Thesis (PhD)
Keywords: ELF, line fishery, effects of line fishing, Great Barrier Reef, exploitation, reef fishes, reef fishes, epinephelines, coral trouts, serranids, Plectropomus leopardus, Cephalopholis cyanostigma, Epinephelus fasciatus, otolith chemistry, otholith morphology, stock structure, stock identification, life histories, harvests, mortality, growth, longevity, sustainability, management, fisheries, spawning, spatial variation, temporal variation, population dynamics, fishing effort
Date Deposited: 01 Dec 2008 00:04
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 0%
07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070402 Aquatic Ecosystem Studies and Stock Assessment @ 0%
06 BIOLOGICAL SCIENCES @ 0%
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